Take any macro volume in the dielectric , when there is no external electric field, dielectric Not polarized, all molecules in this small volume Electric dipole moment The vector sum of is 0, that is 。 When an external electric field exists, the dielectric will be polarized, and the molecular electric dipole moment in this small volume The vector sum of will not be 0, that is 。 The stronger the external electric field, the greater the vector sum of the molecular electric dipole moment. Therefore, we use the vector sum of the molecular electric dipole moment in the unit volume to express the degree of polarization of the dielectric Where It is called electrode strength, and the unit is 。 [1] Since the polarization charge of the dielectric is Dielectric polarization Therefore, there must be some quantitative relationship between the polarization charge and the polarization intensity. We can use a special case to deduce this relationship. As shown in the overview diagram, in a pair of uniform charged planes with an area of S and a distance of d (let the free charge surface densities of the two planes be and The linearity of S is much larger than that of d), and isotropic homogeneous dielectric is filled between them. Due to the effect of electrode, the dielectric plane close to the charged plane will have polarized charges that are different from the free charges in uniform plane distribution, and its surface density is and And there is no residual polarization charge in the medium. So the sum of the electric dipole moment vector of the whole medium is , while the electric polarization intensity is This shows that in a homogeneous isotropic dielectric, the magnitude of the electric polarization intensity is equal to the polarization charge surface density generated by the electrode
Similarly, we use the above special case to analyze the polarization charge and Bound charge , polarization intensity and electric field intensity The electric field intensity generated by free charge is The direction is downward. If the surface density of the polarized charge on the dielectric surface is, the electric field intensity of the electric field excited by the polarized charge is The direction is upward. So the electric field strength in the medium is Utilization , substituted into the above formula: So I got Free charge The relationship between density and polarization charge density is Use relation , the relationship between free charge and polarized charge is The numerical relationship between the electric polarization intensity and the electric field intensity in the dielectric can be obtained
definition Is called the electrical susceptibility of the dielectric. The corresponding vector relationship is The above formula shows that in isotropic dielectric, the electric polarization intensity is proportional to the electric field intensity, and the direction is the same
It should be noted that, Isotropy When a homogeneous dielectric is polarized in an electrostatic field, its relative Permittivity and Electropolarizability Is a constant. However, when it is polarized in a high frequency changing electric field, due to the time required for dielectric polarization, the change of molecular electric dipole moment cannot keep up with the change of electric field Relative permittivity Will fall. So in the high-frequency electric field, the dielectric Relative permittivity It is related to the frequency of the applied electric field. [2] 
